The invention relates to a multilayer focusing spherical lens which can be incorporated in a transceive antenna of a terminal of a remote transceiver system.
The invention also relates to a transceive antenna including a lens of the above kind and a terminal for transmitting and receiving radio signals to and from at least two remote transceiver systems moving at different points in the field of view of said terminal, the terminal including an antenna of the above kind.
The invention applies in particular to systems for transmitting data at a high bit rate to and from a constellation of satellites for public or private, civil or military use, but this application is not limiting on the invention.
More generally, the invention relates to any application requiring a lens of simple structure with which a compact antenna can be obtained.
One solution to the problem of simplifying the structure of the lens in an antenna is to use a single-layer focusing spherical lens, of the kind shown in FIG. 1. Such lenses have the advantage that they are easy to manufacture because they comprise only one layer, and possibly also an index matching layer, as shown.
However, for a given overall size, such lenses have relatively low gain, yielding an antenna efficiency of less than 50%. In the example shown in FIG. 1, even though the various parameters of the lens have been optimized, such as the refractive index, the diameter and the losses by reflection limited by the index matching layer, the gain is still low because of the convergent rays, which represent a loss of energy and disturb the radiation pattern of the antenna in the form of raised secondary lobes. Experience shows that reducing the refractive index increases the focal length and therefore increases the overall volume of the antenna, whereas increasing the refractive index increases ohmic losses without improving the focusing of the lens.
One solution to that problem would be to increase the overall size of the lens to obtain satisfactory gain, for example gain of the order of 31 dB in the applications in question. However, this is not acceptable because it leads to overall size and additional weight which are incompatible with minimizing the overall size and weight of a transceive terminal.
A second solution uses a multilayer Luneberg lens, as shown in FIG. 2. Such lenses comprise a plurality of concentric spherical layers of dielectric constant that decreases continuously from the center towards the edge of the lens. That type of lens has the advantage of total spherical symmetry, which is ideal for producing an antenna with a very wide field of view.
However, for given overall size, such lenses also have relatively low gain, yielding an antenna with efficiency of 50% to 60%. FIG. 2 shows divergence of many rays despite relatively fine sampling of the theoretical law stated by Luneberg. To obtain high efficiency it is necessary to increase the number of layers considerably, which is totally prohibitive in terms of manufacturing cost, especially for mass-market applications.
Finally, U.S. Pat. No. 4,307,404 describes a planar and spherical multilayer antenna design and refers to a spherical artificial structure.
However, the problem addressed in the above document is concerned with interference between different frequencies. Consequently, the beam is deflected for certain frequencies only and the antenna described is therefore not a particularly broadband antenna: the beam is swept mechanically in the same direction for all frequencies compatible with the radiating source.